About 1.85%. Assuming you did a 0.030 overbore, shaved the heads/thinned the gaskets as much as possible (before something hits inside!) to boost compression, matched rings to cylinders/pistons, and all those other little tweaks that they do to race engines...
Maybe ten horsepower. And I mean maybe, you'd be very lucky to get that. And that kind of engine work would run you several grand (despite parts costs being ~0). So lets say at only $1000 (a smokin' deal!), that's $100/hp: and that's highly optimistic! Now, a 3.0L, on the other hand, would get you ~30-40 hp for not much more money: being very conservative, it's $100 per hp. More likely you'd pay $500/hp for boring, etc., and $50/hp for a 3.0L.
It's just not worth it, eh?
